Predicting Thermals with FR4

30019203

FIGURE 3. System Diagram 1

Using finite element analysis, the thermal performance of the

LM3404HV in PSOP-8 and SO-8 was simulated with three

different sized PCBs. These simulations assumed an ambient

temperature of 22°C, a power dissipation of 1W, and a 2-layer

PCB comprised of 1oz copper on top and bottom, separated

by 62mil of FR4. The results are listed below:

θ

PSOP-8

SO-8

1.25" x 1.95" (Eval Board)

55.1

104.8

1" x 1" (Typical Case)

73.0

120.8

0.5" x 0.5" (Worst Case)

240.2

263.7

There is good correlation between the test results and simu-

lation results for the Evaluation Board Case. The strong in-

fluence of the PCB size can be seen by comparing the results

from the demo board to those of the Typical and Worst Cases.

Without enough copper area to spread and dissipate heat, the

advantages of the exposed pad are reduced, and even the

PSOP-8 package will be limited to low power, low ambient

temperature applications.

Predicting Thermals with Metal-

Core PCB

30019204

FIGURE 4. System Diagram 2

Finite element analysis was again used to simulate the per-

formance of the LM3404HV when dissipating 1W in PSOP-8

and SO-8 packages on a 1” square section of MCPCB with

the following composition:

Material

Thickness (mil)

Thermal

Conductivity

(W/m-K)

Metal Core (6061-T6

aluminum)

40

167

Insulator Material

3

2.4

Copper Traces (75%

coverage)

1.4

377

To attain typical operating conditions, the 1” square section

containing the LM3404HV was assumed to be part of a larger

MCPCB measuring 1” x 6”. The other 5 square inches con-

tained 5 high power LEDs, and it was assumed that they

heated the entire board to a uniform temperature, TBD, of 50°

C (representing a large heatsink) and 75°C (representing a

smaller heatsink.) The results for thermal impedance from the

board to the LM3404HV die,

, are listed below:

(°C/W)

T

T

T

T

T

T

T

T

SO-8

70.5

69.2

71.4

70.5

PSOP-

8

9.8

9.2

10.5

10.1

The parameter

can be used to calculate the LM3404HV

junction temperature using the following equation:

Alternatively,

can be used to determine the maximum tol-

erable board temperature by working back from a T

125°C:

Two main conclusions can be drawn from the results. First,

the thermal impedance has little dependence on the board

temperature or the ambient temperature, and instead de-

pends highly on the package used. Second, the PSOP-8

package is again far superior to the SO-8 package when the

DAP is soldered to a mass with low thermal impedance, such

as an MCPCB.

3

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